The Midi File
This is a standard file that represents the musical events in a piece of music. The filename usually has an extension of .mid. It can be used to play a piece of music on midi instruments.
This program uses MidiFiles to play a piece of music, and to allow the chords to be generated algorithmically. A MidiFile is usualy present in the Project folder, along with a ChordFile and sometimes some other files.
There are a large number of these files available on the internet. A google search for "midi files" will return many sites listing thousands of MidiFiles.